Some Geometric Aspects of Towed Air Gun Arrays

Abstract

Abstract Air gun array technology has developed in the 1970's along the general line of producing tuned arrays with source strengths in excess of 100 bar-meters (peak-to-peak) in the seismic bandwidth and with wavelets of front-to-back ratios in excess of 15:1. Most such arrays are towed in a so-called "point-source configuration", for which a significant amount of energy is beamed in such undesirable directions as forward and to each side of the towing vessel. This paper examines several available air gun array geometries; the SLAG array, in which the air gun source array is extended along the axis of a survey line, the SWAG array, in which the air gun source array is extended along an axis perpendicular to the survey line, and the point source array .It is shown how noise problems characteristic of certain offshore areas may be reduced by beaming the array in preferred directions. It is shown that this beaming may improve the achievable signal-to-noise ratio --- both by increasing power input to the water in desirable directions, and by reducing noise arriving from unwanted directions. Air Gun Array Design Criteria (1970's) The conventional design criteria for tuned air gun arrays are:Maximize the peak-to-peak amplitude of the array signature as one would observe at normal incidence in the far-field.Maximize the ratio of the peak amplitude to the residual bubble amplitude (P/B ratio), as observed at normal incidence in the far-field.Assume that, in the seismic bandwidth, typically 10 Hz to 125 Hz, the power spectral density of the array signature is smooth, i.e., the array signature should resemble a band-limited spike.Adjust the depths of source and streamer and the recording system impulse response to place spectral notches at uninteresting frequencies, hence optimizing the frequency band desirable for the geophysical objective. Giles and Johnston (1973) and Nooteboom (1978) discuss these criteria and one example of a modern, high resolution, deep penetration array, reported by Brandsaeter, et. al (1979), has a total volume of 4165 cu. in. with a peak-to-peak amplitude of 49.5 bar-meters and a primary-to-bubble ratio of 16.7. This total array was deployed astern in a surface area 16 m × 20 m, and, while it is evident that some spatial beam forming is taking place, this array is commonly thought of as being in a "point source configuration." Non-Source Generated Noise Implicit in the design criteria mentioned above is the notion that the signal must be large compared to any additive random and incoherent noise which is not source-generated. By simply increasing the source power, such noises as electrical noise, certain types of towing noises, cultural noises and ambient sea noise may be overcome. In addition, this helps overcome the effects of absorption and other frequency-dependent loss mechanisms. Source-Generated Noise The effects, on the stacked section, CDP and shot gathers of such noises as guided waves in near-surface layers, side scatter from reefs, islands, river channels, and reflected refractions are discussed by Larner, et al,198l.